A variety of tests are presently available to measure the quality of unset concrete which is indicative of the strength and quality the concrete will have after it has set. The quantity of water contained in a concrete mix bears directly on the quality and strength of the finished concrete. A standard measure of the quantity of water in a concrete mix can be determined by measuring the slump of the mix. Slump is a measure of the degree to which a volume of unset concrete will settle when allowed to stand.
Typically, enough concrete is gathered from the discharge chute of a concrete mixing truck to fill a testing cone. The testing cone is a hollow frustum of rigid material having an opening at the top and an opening at the bottom. The cone is usually twelve inches high. The open bottom typically has a ten inch diameter and the open top typically has a four inch diameter. Concrete is packed into the cone in three equal layers and is tamped twenty-five strokes per layer. The cone is then topped off with concrete so that the concrete is flush across the top opening. The cone is then twisted gently and raised carefully off the concrete which then sags or slumps. The empty cone is placed next to the concrete so that the difference between the height of the cone and the height of the concrete can be measured. This difference in inches is the slump of the concrete.
This test is both time-consuming and subject to inaccuracies due to human variation. If the cone is not loaded properly or tamped properly an incorrect measurement will result. It often takes five or six minutes to complete the test. If the slump is greater than the known allowable slump for the specific mix in question, then the concrete mix contains too much water and must either be rejected or adjusted. Often, concrete is being discharged while an examiner completes the slump test. If the concrete fails the slump test, the remainder of the load must be rejected.